1. Field of the Invention
The present invention relates to a mobile radio terminal used for, for example, a moving body.
2. Description of the Related Art
In radio communication systems of, for example, a mobile telephone system and a wireless LAN, OFDMA (Orthogonal Frequency Multiple Access) and CDMA (Code Division Multiple Access) are employed as communication schemes. OFDM (Orthogonal Frequency Division Multiplexing) is proof against interference or disturbance and is therefore noticed.
In 3GPP LTE (Long Term Evolution) employing the OFDM (see, for example, Erik Dahlman “Academic Press”, 2007, pp. 277-369, 3GPP TS23.401, 3GPP TS23.402, 3GPP TS36.300), entire data over a network are handled as packets, and the packet communication scheme using VoIP (Voice over IP) is employed as the speech communication.
In the 3GPP LTE, however, the speech communication based on the line switching system which has been employed in the 2G/3G system is not quite used, but performing the speech communication by using the 2G/3G system of the existing line switching system at the time of transmitting and receiving speech as circumstances demand, is studied.
In the currently studied system, if a mobile station in a standby status receives speech over an LTE network (hereinafter called E-UTRAN network), the mobile station supplies the speech to a 3G network (hereinafter called 1×TT network) and then becomes in a communication status with the 1×TT network in predetermined steps.
In the above steps, however, information indicating the system of the transition target (1×TT network) and a frequency thereof are merely described in Redirection Information Element notified to the mobile station from the E-UTRAN network. For this reason, the mobile station needs to perform a process of capturing the system from the first step, over the 1×TT network, and it takes much time to establish a communication link and much delay time occurs before the communication starts. Since 2 to 3 seconds are generally required for this process, a delay which is more than about ten times as great as that in a general case occurs in consideration of the matter that it takes 100 ms to establish the conversation in the receiving status over the 1×TT network.
Upon switching the E-UTRAN network to the 1×TT network, preliminarily measuring the receiving environment of the 1×TT network serving as the transition target and performing the transition by CCO (Cell Change Order) which informs the E-UTRAN of the measurement result can be considered. In this process, however, the 1×TT is not supported under the current 3GPP standards.
The conventional mobile radio terminal corresponding to a plurality of communication system has a problem that it takes much time to transit to the CDMA communication system under instructions from the LTE communication system.